1. Technical Field
The present invention relates to an insulation measuring apparatus and, more particularly, an insulation measuring apparatus capable of measuring an insulation resistance by using a flying capacitor, for example.
2. Background Art
In order to supply an electric power to electrical equipments such as a light illuminating system, an air conditioner, etc. or to give a charge of electricity to the equipments, conventionally an automobile is equipped with a battery. The recent automobiles so utterly depend on the electric power that it would be no exaggeration to say that the automobile cannot work without the electric power.
Also, restrictions on an exhaust gas (emission control) are getting tighter from a viewpoint of the global warming measure, and the like. In answer to this tendency, a part of automobile manufacturers already puts hybrid vehicles, which employ both an engine and a battery as a driving power, onto the market to reduce consumption of fuel. Also, this tendency is accelerating more and more, and now the battery is being used as the driving power in a huge number of automobiles.
Under such circumstances, electric power management becomes more important for the automobile manufacturer than ever before. In particular, the voltage that is very higher than the conventional voltage is employed in the situation that a high-power battery is equipped for the driving purpose. Therefore, it is highly possible that an electric shock is caused when the insulation is degraded. For this reason, monitoring of an insulation state becomes more important nowadays than heretofore.
As the technology to check an insulation state, various technologies have been introduced. For example, there is an insulation measuring circuit of flying capacitor type (see JP2004-170103A). FIG. 1 shows a circuit diagram of an insulation measuring circuit 110 disclosed in JP2004-170103A. Also, FIG. 2 is a time chart explaining the data reading by AD of a decision control unit 130 (referred to as an “AD reading” hereinafter).
The insulation measuring circuit 110 includes a detecting circuit 120 and the decision control unit 130, and detects an insulation state of a power supply V. The detecting circuit 120 has a capacitor (flying capacitor) C11 that is set in a floating state from a ground electric potential, and also has first to sixth resistors R11 to R16 and first to fifth switches SW11 to SW15. Each of the first to fifth switches SW11 to SW15 is composed of an optical MOSFET (Metal-Oxide-Semiconductor Field-Effect-Transistor), for example. It is desired that the first resistor R11 and the second resistor R12 should have a very high resistance value such that an insulation of the system is not decreased even in the case of short-circuit failure of the switch.
Then, the decision control unit 130 turns ON the first and second switches SW11, SW12 such that a path consisting of the first switch SW11, a first diode D11, the first resistor R11, the capacitor C11, and the second switch SW12 is formed from the positive electrode side of the power supply V to the negative electrode side and that a voltage of the power supply V is set to the capacitor C11 (this voltage is referred to as a “high voltage V10” hereinafter). Then, when the first and second switches SW11, SW12 are turned OFF and the third and fourth switches SW13, SW14 are turned ON (T11 in FIG. 2), a closed circuit consisting of the capacitor C11, a second diode D12, the second resistor R12, the third switch SW13, the third resistor R13, the fourth resistor R14, and the fourth switch SW14 is constructed. A voltage divided by the second resistor R12, the third resistor R13, and the fourth resistor R14 is input into the decision control unit 130 (input port AD) through the sixth resistor R16 and measured asV10×R13/(R12+R13+R14).
In this case, a cathode of a third diode D13 is connected in the middle of the path between the sixth resistor R16 and the input port AD and an anode of the third diode D13 is connected to a ground electric potential. When the measurement is finished, the third switch SW13 is turned OFF and also the fifth switch SW15 acting as the discharge switch is turned ON (T12 in FIG. 2). Thus, an electric charge on the capacitor C11 is discharged through the fifth resistor R15 and the fourth resistor R14 of the fourth switch SW14 (T12 to T13 in FIG. 2). When a discharge is finished (T13 in FIG. 2), the fourth switch SW14 is turned OFF.
Then, the decision control unit 130 charges the capacitor C11 in a state that one terminal of the capacitor C11 is grounded via the fourth resistor R14, and then measures the voltage that is set on the capacitor C11. Concretely, the decision control unit 130 turns ON the first switch SW11 and the fourth switch SW14. According to the ON state of these switches, a path that consists of a negative-electrode side ground fault resistor RLn, the power supply V, the first switch SW11, the first diode D11, the first resistor R11, the capacitor C11, the fourth switch SW14, and the ground potential G is constructed from a ground potential G. At this time, a charging voltage VC11 (negative-electrode side ground fault resistor voltage) is set on the capacitor C11. Then, when the first switch SW11 is turned OFF and the third switch SW13 is turned ON, a divided voltage of the charging voltage VC11 being set on the capacitor C11 similarly to the above is input into the decision control unit 130 through the sixth resistor R16 and measured asVC11×R13/(R12+R13+R14).
When the measurement is finished, the third switch SW13 is turned OFF and the fifth switch SW15 is turned ON. Thus, an electric charge on the capacitor C11 is discharged through the fifth resistor R15 and the fourth resistor R14.
Then, the decision control unit 130 turns ON the second switch SW12 and the third switch SW13. According to the ON state of these switches, a path that consists of the third resistor R13, the third switch SW13, the first diode D11, the first resistor R11, the capacitor C11, the second switch SW12, the power supply V, a positive-electrode side ground fault resistor RLp, and the ground potential G is constructed from the ground potential G. At this time, a charging voltage VC12 (positive-side ground fault resistor voltage) is set on the capacitor C11. Then, when the second switch SW12 is turned OFF and the fourth switch SW14 is turned ON, a divided voltage of the charging voltage VC12 being set on the capacitor C11 similarly to the above is input into the decision control unit 130 through the sixth resistor R16 and measured asVC12×R13/(R12+R13+R14).
When the measurement is finished, the third switch SW13 is turned OFF and the fifth switch SW15 is turned ON. Thus, an electric charge on the capacitor C11 is discharged through the fifth resistor R15 and the fourth resistor R14.
In turn, the decision control unit 130 converts the measured voltage into the insulation resistance based on a computational expression (VC11+VC12)/V10, and then detects how a ground fault resistor RL is detected, by referring to a predetermined table. When the detected ground fault resistor RL is less than a predetermined threshold RLy, the decision control unit 130 decides that the insulation is degraded, and gives a predetermined alarm.
The ON states of the third switch SW13 and the fourth switch SW14 are kept while the decision control unit 130 is reading the AD. In this event, a detecting accuracy is lowered when the electric charge of the capacitor C11 is discharged. Therefore, time constants of the second to fourth resistors R12 to R14 (a resultant time constant of R12+R13+R14) must be set to a value that is large enough to prevent the discharge during the AD reading. Meanwhile, the electric charge of the capacitor C11 must be discharged after the measurement is finished. For this purpose, the fifth resistor R15 and the fifth switch SW15 are provided to constitute a discharging circuit, so that the electric charge on the capacitor C11 remained after the finishing of measurement is discharged quickly by turning ON simultaneously the fourth switch SW14 and the fifth switch SW15.
Here, the discharging circuit consisting of the fifth switch SW15 and the fifth resistor R15 needs the optical MOSFET as the insulation element with a high withstand voltage. Thus, an increase in cost is brought about, and another approach is requested. Also, when the discharging circuit mentioned above is newly provided, an area of the high voltage circuit is extended. Therefore, the more cautious design/configuration are demanded from a viewpoint of safety, and thus there exists such a problem that an increase in cost is also brought about.